The present invention relates to methods of, and apparatus for jamming radio communication systems and in particular to what are known as "leave-behind jammers". Such devices are designed for military use, being left during strategic withdrawal from an area so that after the area is over-run by hostile forces the jammers disrupt communications systems in the vicinity To be effective many such jammers need to be deployed in order to cover the many transmissions likely in a complex communications system and to provide sufficient redundancy for jamming still to be effective even after a proportion of the jammers have been detected and disabled. Ideally therefore the jammers should be rugged, portable and sufficiently inexpensive to be regarded as expendable.
Known methods of jamming fall broadly into two categories: "smart set-on" and "barrage". In the former the jamming device is designed to detect a tranmission from a station local to the jamming device and to transmit a high power jamming signal concurrently with the local transmission. In the latter method the jammer transmits jamming signals continuously over a wide band to jam any reception by a local receiving station. Both these methods require the use of high power outputs and since leave-behind jammers must necessarily contain their own power supplies these high power requirements have been a major obstacle to the achievement of reductions in size and cost. It is known to have a jammer of the former type, i.e. a "smart set-on device" comprising receiving means for receiving a local radio transmission, frequency determining means for determining the frequency of the local radio transmission, and jamming signal transmitting means for transmitting a jamming signal at a frequency controlled in accordance with the frequency determined by the frequency determining means at a time subsequent to the reception of the local radio transmission by the receiving means. British Pat. Nos. 1,278,771 and 1,450,761 disclose examples of such jammers designed to jam radio transmissions in which short messages of typically one tenth of a second duration are transmitted. The jammer is designed to transmit a jamming signal concurrently with the detected transmission. The jammer has to switch rapidly between searching for transmissions and the transmission of a jamming signal in a time-scale of a few milliseconds in order to jam effectively the detected short duration signal before it ceases. This jammer requires sophisticated circuitry in both its detection and jamming stages in order to function at the high speeds required.
U.S. Pat. No. 4,214,208 discloses a further example of such a jammer. In common with other known jamming devices the jammer disclosed in this patent transmits a jamming signal concurrently with the victim signal. In this case because the victim signal is a keyed continuous wave signal of the type used in radio telegraphy the jamming signal is also keyed, having a complementary waveform with each pulse timed to follow immediately upon an individual corresponding pulse of the victim signal. As with the other devices discussed above the jamming signal has to be of substantially the same power level as the victim signal in order to be effective.